This section is badly out of date. To gain
a better view of traditional vs. current estimates of metazoan phylogeny, I
recommend that you instead view these
diagrams for a comparison ofolder
views of animal relationships with a more up-to-datetree
of animal relationships based on DNA sequence.For
an overview of the early fossil record of animals throughtime
click here,
or see this
article for a more detailedoverview
including recent fossil and developmentaldiscoveries.

Recent molecular studies have surprisingly
found evidence for a more basal position of Silicea within metazoans. By this
view, Calcarea are sister taxon of Eumetazoa (all metazoans except for sponges),
so "Porifera" is paraphyletic and must be abandoned. If this phylogenetic
hypothesis is accurate, it strengthens the notion that the ancestor of all
metazoans was sponge-like. It also suggests that typical textbook emphasis
on a grade of increasing complexity within sponges (e.g., Fig. 12-5), from
asconoid, to syconoid, to leuconoid, has nothing to do with the origin of
eumetazoans from sponge-like ancestors. This is because our closest relatives,
calcareans, generally have the more simple asconoid body type.

B. How a sponge works

Aquatic animals

Can be soft-bodied but most firm to woody
 internal skelton of many tiny calcareous or siliceous spicules
 many also have tough sponge fibers

Sponges are perforated by a nework of canals
 water moves to chambers
 moves to chambers lined with "collar" cells (Choanocytes)
 exits through a corridor to one or more large oscula

Sponges are highly specialized to filter feed.
 One sponge can filter multiple liters of water

Sponges have tissues
 Complex array of cell types
 No organs or nervous system
Only two "systems"
canals/chambersskeleton

See Fig. 12-10: Note how the choanocyte cell (collar
cell) traps food particles through the action of a beating flagellum, drawing
water through collar microvilli. Note that incurrent flow is through
tiny ostia pores, whereas relatively fewer collar cells share exhalent
oscula pores. The sponge body is more or less complicated (Fig. 12-5).
Note that there are loosely organized "tissue layers" in a sponge,
with different types of cells, including archaeocytes, pinacocytes,
and collencytes, all suspended in a gelatinous mesohyl matrix,
typically with a skeletal framework of spicules, and usually also spongin
fibers (Fig. 12-9). (Bath sponges fortunately have spongin but no spicules.)

C. What did sponges evolve from?

Note that sponges feed by cellular digestion. They have no
gut. A gut is a synapomorphy of Eumetazoa, the clade of all metazoans besides
sponges. In this regard sponges are primitive. (In other aspects they are
highly specialized.) Sponge choanocyte cells very closely resemble the "protist"
organisms we covered in Chapter 11 called choanoflagellates
(e.g., Codosiga, Fig. 11-14). For descriptions of some other choanoflagellates,
see Monosiga
or Salpingoeca.
Despite their resemblance to choanoflagellates, sponges are more closely related
to us (and other eumetazoans) than they are to choanoflagellates. The evidence
for this statement is the synapomorphies
we share with sponges, lacking in choanoflagellates. These derived similarities
are most parsimoniously explained by the hypothesis that we share a common
ancestor with sponges that is more recent than the one either of us shares
with choanoflagellates.